To address issue 149,150 and 155 for simple deploy, Capistrano error handling 

This change addresses issue https://github.com/intuit/simple_deploy/issues/150

Prior to this change a user wasn't given feedback if the command executed successfully. If command passed was valid or invalid, the response was the same.

After the code change if a valid command was executed, an info message is displayed to stdout, if an invalid command was executed, an error message is displayed to stdout.

Prior to code change (invalid command):
```
$ simple_deploy execute -e default -n kyoung-testapp08 -c uptme
2013-02-13 15:51:00 -0800 INFO : Setting command: 'uptme'.
$ 
```
After code change (valid command):
```
$ simple_deploy execute -e default -n kyoung-testapp08 -c uptime
2013-02-13 15:51:53 -0800 INFO : Setting command: 'uptime'.
2013-02-13 15:51:55 -0800 INFO : Command executed against instance(s) successfully.
$ 
```
After code change (invalid command):
```
$ simple_deploy execute -e default -n kyoung-testapp08 -c uptme
2013-02-13 15:55:13 -0800 INFO : Setting command: 'uptme'.
2013-02-13 15:55:14 -0800 ERROR : Command executed against instances unsuccessfully.
```